Add Yara yls support (#4886)

This adds Yara support to ALE using Avast's language server for Yara:
https://avast.github.io/yls/

A ".git" folder is used to determine the project_root so this means the
Yara rules must be in a git repo for the integration to work.

The server only have 1 optional argument (-v, --verbose). Since this is
the case, no additional configuration options are available.

---------

Co-authored-by: w0rp <w0rp@users.noreply.github.com>

test/linter/test_yara_yls.vader

@@ -0,0 +1,13 @@
+Before:
+  call ale#assert#SetUpLinterTest('yara', 'yls')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The default executable path should be correct):
+  AssertLinter 'yls', ale#Escape('yls') . ' -v'
+
+Execute(The project root should be detected correctly):
+  call ale#test#SetFilename('../test-files/yara/dummy.yar')
+
+  AssertLSPProject ale#path#Simplify(g:dir . '/../..')